1. Field of the Invention
The invention relates to an optical element, an objective optical element and an optical pickup device in which a light beam is converged on an information record plane of an optical information recording medium.
2. Description of Related Art
Recently, because a shorter wavelength red color laser has been practically used, a digital versatile disc (DVD) which is a large capacity and high intensity optical information recording medium (also called an optical disc) and has the approximately same size as a compact disc (CD), has been produced.
In a recording and reproducing device for the DVD, when a semiconductor laser for 650 nm wavelength light is used, a numerical aperture NA of an objective lens on the side of an optical disc is set within an range from 0.6 to 0.65. In the DVD, a track pitch of 0.74 μm and a shortest bit length of 0.4 μm are set to record data in high intensity as compared with the CD having the track pitch of 1.6 μm and the shortest bit length of 0.83 μm. Further, in the DVD, to suppress coma occurring when an optical disc is inclined with respect to an optical axis, the thickness of a protect substrate is set to 0.6 mm which is half of that of the CD.
Further, in addition to CD and DVD, various types of optical discs such as compact disc-recordable (CD-R), CD-rewritable (CD-RW), video disc (VD), mini disc (MD) and magneto-optical disc (MO) corresponding to different wavelengths of light emitted from light sources and different values of transparent substrates in thickness have been put on the market.
Moreover, the wavelength of light emitted from the semiconductor laser has been shortened. Therefore, a high intensity optical disc (hereinafter, called “high intensity DVD”) having a protect substrate of a thickness of 0.1 mm and used with a blue-violet semiconductor laser emitting a light having a wavelength of about 400 nm and an objective lens of the heightened image-side numerical aperture of approximately 0.85 has been researched and developed. Further, a high intensity DVD having a protect substrate of a thickness of 0.6 mm and used with an objective lens of the heightened image-side numerical aperture of approximately 0.65 has been researched and developed.
Furthermore, various types of optical pickup devices respectively having an objective lens to converge light beams of different two wavelengths on each information record plane of two types of optical discs respectively, that is, the interchangeable optical pickup devices are proposed in Published Unexamined Japanese Patent Applications (Tokukaihei) No. 2001-93179 (first patent literature), No. 2000-81566 (second patent literature), No. H9-54973 (1997) (third patent literature), No. H9-306018 (1997) (fourth patent literature) and PCT International Publication No. 98/19303 (fifth patent literature).
In each of the first and second patent literatures, an optical pickup device having a diffraction optical element with a plurality of diffracting ring-shaped zones formed in a serrate shape in section is disclosed.
In this device, for example, a high diffraction efficiency is obtained by setting the blaze depth of the diffracting ring-shaped zone so as to heighten the diffraction efficiency of the diffracted light having a prescribed order for each of two light beams having different wavelengths, and the diffracted light is converged on a prescribed optical disc. Accordingly, the recording and reproduction of information is performed for two types of optical discs by using only one objective lens.
In the device disclosed in the first patent literature, each of the diffracted light beams having different diffraction orders, which are caused by diffracting two light beams having different wavelengths, is converged on the prescribed optical disc through the objective lens. Because the diffraction orders are determined according to the difference between the wavelengths, the combination of the diffraction orders is limited, and the correction of the axial achromatic aberration in each of the two types of optical discs and the correction of the spherical aberration caused by the change of temperature are limited.
Further, in the device disclosed in the second patent literature, each of the diffracted light beams having the same diffraction order, which are caused by diffracting two light beams having different wavelengths, is converged on the prescribed optical disc through the objective lens.
Diffraction efficiencies of diffracted light beams of diffraction orders ranging from minus second order to plus second order are shown in FIG. 8 in a wavelength range of light source from 350 nm to 800 nm when the diffracted light beams are obtained in a diffraction optical element disclosed in each of the first and second patent literatures and formed in a well-known serrate shape in section.
In this diffraction optical element, the diffraction efficiency of the −1st order diffracted light is set to approximately 100% in the wavelength of around 400 nm. Therefore, in the wavelength of around 650 nm used, for example, for the DVD, the diffraction efficiency of the −1st order diffracted light is lowered to a value ranging from 50% to 60%. In this case, when the diffraction optical element is used for two types of optical discs corresponding to wavelengths largely different from each other, a problem has arisen that an amount of light converged on one optical disc is insufficient.
The reason that the diffraction efficiency for a light beam having a wavelength other than a specific wavelength is lowered will be described. The size of the blaze of the diffraction optical element is set to give an optical path difference equal to the integral multiple of the specific wavelength to the light beam having the specific wavelength when the light beam having the specific wavelength passes through the blaze. Therefore, when a light beam having a wavelength other than the specific wavelength passes through the blaze of the diffraction optical element, an optical path difference not equal to the integral multiple of the wavelength of the light beam is given to the light beam. Therefore, the diffraction efficiency for the light beam is lowered.
In each of the third and fourth patent literatures, an optical pickup device having both a plane hologram optical device and a refraction type objective lens separated from each other is disclosed.
In this device, a light beam having one of two wavelengths is transmitted through the hologram optical device and is converged on a prescribed disc through the objective lens. Further, another light beam having the other wavelength is transmitted through the hologram optical device while being diffracted and diverging, and a −1st order diffracted light among various types of diffracted light is converged on the prescribed disc through the objective lens. Therefore, the recording and reproduction of information for two types of optical discs can be performed by using one objective lens.
Further, in the fifth patent literature, an optical pickup device having both an optical element and a refraction type objective lens separated from each other is disclosed. The optical element has both a hologram formed in a region (central region) arranged around the optical axis and a diffraction grating formed in the periphery (peripheral region) of the central region.
In this device, a light beam having the wavelength of 635 nm is transmitted through the central region, and a light beam having the wavelength of 780 nm is diffracted in the central region. Further, the light beam having the wavelength of 635 nm is transmitted through the peripheral region, and the light beam having the wavelength of 780 nm is diffracted in the peripheral region to substantially interrupt the light.
Therefore, all the light beam having the wavelength of 635 nm is incident on the objective lens, and a partial light beam having the wavelength of 780 nm and passing through the central region is diffracted while diverging and is incident on the objective lens. Accordingly, the recording and reproduction of information for two types of optical discs can be performed by using one objective lens.
However, in the devices disclosed in the third, fourth and fifth patent literatures, one light beam having one wavelength is diffracted in the hologram optical element, and the light beam is converged on the prescribed optical disc through the objective lens. The other light beam having the other wavelength is transmitted through the hologram optical element, and the light beam is converged on the prescribed optical disc through the objective lens.
Here, the diffraction efficiency is determined according to the number of steps of concave and convex portions formed on the hologram optical element. When the diffraction efficiency of the transmitted light approximately equals to 100%, the diffraction efficiency of the diffracted light is limited. For example, the diffraction efficiency equals to around 81% in the four-step structure, around 88% in the five-step structure and around 91% in the six-step structure. When the device is used for the recording of information, a problem has arisen that an amount of diffracted light is not sufficient. When the number of steps of the concave and convex portions in the hologram optical element is increased to increase the amount of diffracted light, a problem has arisen that it is difficult to make a metallic mold of the hologram optical element.
Further, in the devices disclosed in the third, fourth and fifth patent literatures, because the hologram optical element and the objective lens are separately arranged, a problem has arisen that a size of the device is inevitably enlarged.
Moreover, because the light beam is diffracted and diverges in the hologram optical element, an amount of light used for the recording and reproduction becomes low. Therefore, a problem has arisen that an amount of light converged on the optical disc is insufficient.
Furthermore, because the hologram optical element and the objective lens are positioned away from each other, the decentering is caused, or the image height characteristic is degraded. As a result, a problem has arisen that axial achromatic aberration occurs or spherical aberration (hereinafter, called “temperature characteristic aberration”) occurs due to the change of temperature exceeding a changing degree expected in the design.
Still furthermore, because the hologram optical element formed on a flat plate is used, the number of concave and convex portions (interference fringe pattern) formed stepwise on the surface of the hologram optical element is increased. Therefore, a problem has arisen that the manufacturing of the hologram optical element is complicated.